Results 1 - 10 of 12686
Results 1 - 10 of 12686. Search took: 0.036 seconds
|Sort by: date | relevance|
[en] The priority of V.A. Kistyakovskii in the discovery of hydration was substantiated. The history of D.I. Mendeleev’s chemical theory of hydrates and S. Arrhenius’ physical theory of electrolytic dissociation was considered; the Kistyakovskii concept spans these theories. The role of I.A. Kablukov in hydration research and modern aspects of the theory were discussed.
[en] Using first-principles calculations, we show that the formation of carbohydrates directly from carbon and water is energetically favored when graphene is subjected to an unequal chemical environment across the two sides, with a difference in the chemical potential of protons and hydroxyl groups. The resultant carbohydrate structure is two-dimensional (2D), with the hydrogen atoms exclusively attached on one side of the graphene and the hydroxyl groups on the other side, the latter forming a herringbone reconstruction that optimizes hydrogen bonding. We show that graphene undergoes a metal-insulator transition upon hydration that is readily detectable from the significant shift in the vibration spectrum. The hydrate form of graphene offers new applications for graphene in electronics, either deposited on a substrate or in solution.
[en] This paper is intended to determine the appropriate conditions for replacing CH4 from NGH with CO2. By analyzing the hydration equilibrium graphs and geotherms, the HSZs of NGH and CO2 hydrate, both in permafrost and under deep sea, were determined. Based on the above analysis and experimental results, it is found that to replace CH4 from NGH with gaseous CO2, the appropriate experimental condition should be in the area surrounded by four curves: the geotherm, (H-V)CO2, (L-V)CO2 and (H-V)CH4, and to replace CH4 from NGH with liquid CO2, the condition should be in the area surrounded by three curves: (L-V)CO2, (H-L)CO2 and (H-V)CH4. For conditions in other areas, either CO2 can not form a hydrate or CH4 can release little from its hydrate, which are not desirable results
[en] Clathrate hydrates are non-stoichiometric compounds that form when water and certain low molecular weight hydrocarbons coexist at high pressures and low temperatures. The majority of the earth hydrocarbons are in the hydrate phase and are primarily located along the ocean bottoms and to a lesser degree in the permafrost regions. In addition, hydrate formation is induced in undersea gas transmission lines and causes costly pipeline plugs and requires expensive inhibition measures to be taken. Therefore, both a stick and a carrot motivate hydrate research. They are a costly and dangerous nuisance to the oil and gas industry and represent a tremendous, untapped energy resource of the future. The formation mechanism of clathrate hydrate formation has always been shrouded in mystery, and an ongoing debate has ensued as to whether their formation is a bulk or surface phenomenon. Molecular dynamics simulation and fractal modeling suggest that this may be an irrelevant issue and that two independent factors contribute to the symmetrical ordered structure of clathrate hydrates: hydrophobic hydration of hydrocarbon molecules in water and formation of linked cavities as these small clusters aggregate. (Author)
[en] Abstarct: Fluorescence quenching of 1-hydroxynaphthalene-4-sulfonate by water has been studied in methanol–water and ethanol–water mixtures. The fluorescence quenching has shown a significant deviation from the Stern–Volmer linearity at higher concentrations of water due to dynamic quenching along with transient effects and instantaneous quenching. Time-resolved fluorescence decay of the conjugate base of 1-hydroxynaphthalene-4-sulfonate in methanol–water mixture indicates the need of only one water molecule to hydrate the conjugate base aftermath of the dissociation of the photoacid. -- Highlights: • A strong photoacid shows positive deviation from Stern–Volmer plot. • It is explained in terms of transient effects and instantaneous quenching. • The deviation depends on the fluorescence lifetime and quencher concentration. • The Stern–Volmer and quenching constants have been obtained experimentally. • Only one water molecule is needed to hydrate the naphthalate ion
[en] Previous studies have used neutron diffraction to elucidate the hydration of the ceramide and the phosphatidylcholine headgroup in solution. These solution studies provide bond-length resolution information on the system, but are limited to liquid samples. The work presented here investigates how the hydration of ceramide and phosphatidylcholine headgroups in a solution compares with that found in a lipid bilayer. This work shows that the hydration patterns seen in the solution samples provide valuable insight into the preferential location of hydrating water molecules in the bilayer. There are certain subtle differences in the distribution, which result from a combination of the lipid conformation and the lipid-lipid interactions within the bilayer environment. The lipid-lipid interactions in the bilayer will be dependent on the composition of the bilayer, whereas the restricted exploration of conformational space is likely to be applicable in all membrane environments. The generalized description of hydration gathered from the neutron diffraction studies thus provides good initial estimation for the hydration pattern, but this can be further refined for specific systems.
[en] Highlights: ► Solubility of fluoromethane in water as a function of (T, p) was observed. ► Liquid + hydrate + vapor phase equilibrium of the system was observed. ► Ice + hydrate + vapor equilibrium of the system was observed. ► Dissociation enthalpies were found for both equilibria. ► Stoichiometry of fluoromethane hydrate was determined. - Abstract: A study of the (fluoromethane + water) system was conducted at temperatures between 255 K and 298 K, and pressures from 0.09 to 2.6 MPa. The solubility of fluoromethane in liquid water was measured from 280 K to 298 K, at pressures up to the hydrate formation pressure. The p–T behavior of the liquid + hydrate + vapor (LHV) three-phase equilibrium was measured from 273 K to 295 K. The p–T behavior of the ice + hydrate + vapor (IHV) three-phase equilibrium was measured from 251 K to 273 K. The intersection of the LHV and IHV curves was used to find the lower quadruple point, Q1, at T = 272.55 K and p = 0.2442 MPa. Solubility-corrected enthalpies of dissociation were determined at the lower quadruple point using the Clapeyron equation. The de Forcrand method was used to determine the hydration number of the hydrate at Q1. The results show that not all of the cages in the SI hydrate structure are filled.
[en] The B3LYP/6-31++G** method was applied to investigate the structure and property of dihydrated alanine complexes. 3 proton transfer reactions were found, the reactants are all more stable than the products. There are 2 kinds of patterns of proton transfer, one is 'direct proton transfer', the other is 'water-bridge' proton transfer. There is no energy barrier in the reverse reaction process of 'direct proton transfer', so the products can't exit steady, in other words the 'direct proton transfer' can't occur actually. The forward and reverse reaction energy barrier of 'water-bridge' proton transfer is respectively 6.47 and 1.43 kcal·mol-1, the forward and reverse reaction energy barrier of hydrogen bond transfer is respectively 2.60 and 1.63 kcal·mol-1. (author)
[en] In this paper empirical relationships for the estimation of enthalpies of formation of simple hydrates of transition metal cations, (Cr2+, Fe2+, Mn2+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+) and UO22+ are derived and tested. (author)
[en] Highlights: → The solubility of difluoromethane in water measured and fitted. → The (liquid + hydrate + vapor) equilibrium was determined and fitted. → The (ice + hydrate + vapor) equilibrium was determined and fitted. → The hydrate dissociation enthalpy and hydration number was determined. → Incomplete hydrate cage filling was observed. - Abstract: A study of the (difluoromethane + water) system was conducted at temperatures between (255 and 298) K, and pressures from (0.06 to 1.30) MPa. The solubility of difluoromethane in liquid water was measured from (280 to 298) K, at pressures up to the hydrate formation pressure. The (p, T) behavior of the (liquid + hydrate + vapor) three-phase equilibrium was measured from (274 to 292) K. The (p, T) behavior of the (ice + hydrate + vapor) three-phase equilibrium was measured from (257 to 273) K. Solubility-corrected enthalpies of dissociation were determined at the lower quadruple point (Q1) using the Clapeyron equation. The de Forcrand method was used to determine the hydration number of the hydrate at Q1. The results show that not all of the cages in the SI hydrate structure are filled.